fix: make credential_config, metadata, next_review_date optional in agent creation

These fields are not in the dashboard's Register Agent form but were
required by the Zod schema. Now they default to null/empty array,
matching the Prisma schema where they're already nullable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: VladUZH

packages/shared/src/schemas/agent.schema.ts

@@ -49,6 +49,11 @@ export const AgentCreateSchema = AgentSchema.omit({
   created_at: true,
   updated_at: true,
   lifecycle_state: true,
+}).extend({
+  authorized_integrations: z.array(AuthorizedIntegrationSchema).default([]),
+  credential_config: z.record(z.string(), z.unknown()).nullable().default(null),
+  metadata: z.record(z.string(), z.unknown()).nullable().default(null),
+  next_review_date: z.string().datetime().nullable().default(null),
 });
 
 export type AgentCreateInput = z.infer<typeof AgentCreateSchema>;